Light emitting and lasing semiconductor methods and devices

ABSTRACT

The invention is applicable for use in conjunction with a light-emitting semiconductor structure that includes a semiconductor active region of a first conductivity type containing a quantum size region and having a first surface adjacent a semiconductor input region of a second conductivity type that is operative, upon application of electrical potentials with respect to the active and input regions, to produce light emission from the active region. A method is provided that includes the following steps: providing a semiconductor output region that includes a semiconductor auxiliary layer of the first conductivity type adjacent a second surface, which opposes the first surface of the active region, and providing the auxiliary layer as a semiconductor material having a diffusion length for minority carriers of the first conductivity type material that is substantially shorter than the diffusion length for minority carriers of the semiconductor material of the active region.

PRIORITY CLAIM

Priority is claimed from U.S. Provisional Patent Application No.61/403,748, filed Sep. 21, 2010, and said U.S. Provisional PatentApplication is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to methods and devices for producing lightemission and laser emission in response to electrical signals. Theinvention also relates to methods and devices for producing lightemission and laser emission from semiconductor devices with improvedefficiency and speed, and to increasing light output from semiconductorlight-emitting devices.

BACKGROUND OF THE INVENTION

A part of the background hereof lies in the development ofheterojunction bipolar transistors which operate as light-emittingtransistors and transistor lasers. Reference can be made, for example,to U.S. Pat. Nos. 7,091,082, 7,286,583, 7,354,780, 7,535,034 and7,693,195; U.S. Patent Application Publication Numbers US2005/0040432,US2005/0054172, US2008/0240173, US2009/0134939, US2010/0034228,US2010/0202483, and US2010/0202484; and to PCT International PatentPublication Numbers WO/2005/020287 and WO/2006/093883. Reference canalso be made to the following publications: Light-Emitting Transistor:Light Emission From InGaP/GaAs Heterojunction Bipolar Transistors, M.Feng, N. Holonyak, Jr., and W. Hafez, Appl. Phys. Lett. 84, 151 (2004);Quantum-Well-Base Heterojunction Bipolar Light-Emitting Transistor, M.Feng, N. Holonyak, Jr., and R. Chan, Appl. Phys. Lett. 84, 1952 (2004);Type-II GaAsSb/InP Heterojunction Bipolar Light-Emitting Transistor, M.Feng, N. Holonyak, Jr., B. Chu-Kung, G. Walter, and R. Chan, Appl. Phys.Lett. 84, 4792 (2004); Laser Operation Of A Heterojunction BipolarLight-Emitting Transistor, G. Walter, N. Holonyak, Jr., M. Feng, and R.Chan, Appl. Phys. Lett. 85, 4768 (2004); Microwave Operation AndModulation Of A Transistor Laser, R. Chan, M. Feng, N. Holonyak, Jr.,and G. Walter, Appl. Phys. Lett. 86, 131114 (2005); Room TemperatureContinuous Wave Operation Of A Heterojunction Bipolar Transistor Laser,M. Feng, N. Holonyak, Jr., G. Walter, and R. Chan, Appl. Phys. Lett. 87,131103 (2005); Visible Spectrum Light-Emitting Transistors, F. Dixon, R.Chan, G. Walter, N. Holonyak, Jr., M. Feng, X. B. Zhang, J. H. Ryou, andR. D. Dupuis, Appl. Phys. Lett. 88, 012108 (2006); The Transistor Laser,N. Holonyak and M Feng, Spectrum, IEEE Volume 43, Issue 2, February2006; Signal Mixing In A Multiple Input Transistor Laser Near Threshold,M. Feng, N. Holonyak, Jr., R. Chan, A. James, and G. Walter, Appl. Phys.Lett. 88, 063509 (2006); and Collector Current Map Of Gain AndStimulated Recombination On The Base Quantum Well Transitions Of ATransistor Laser, R. Chan, N. Holonyak, Jr., A. James, and G. Walter,Appl. Phys. Lett. 88, 14508 (2006); Collector Breakdown In TheHeterojunction Bipolar Transistor Laser, G. Walter, A. James, N.Holonyak, Jr., M. Feng, and R. Chan, Appl. Phys. Lett. 88, 232105(2006); High-Speed (/spl ges/1 GHz) Electrical And Optical Adding,Mixing, And Processing Of Square-Wave Signals With A Transistor Laser,M. Feng, N. Holonyak, Jr., R. Chan, A. James, and G. Walter, PhotonicsTechnology Letters, IEEE Volume: 18 Issue: 11 (2006); Graded-BaseInGaN/GaN Heterojunction Bipolar Light-Emitting Transistors, B. F.Chu-Kung et al., Appl. Phys. Lett. 89, 082108 (2006); Carrier LifetimeAnd Modulation Bandwidth Of A Quantum Well AlGaAs/InGaP/GaAs/InGaAsTransistor Laser, M. Feng, N. Holonyak, Jr., A. James, K. Cimino, G.Walter, and R. Chan, Appl. Phys. Lett. 89, 113504 (2006); Chirp In ATransistor Laser, Franz-Keldysh Reduction of The Linewidth Enhancement,G. Walter, A. James, N. Holonyak, Jr., and M. Feng, Appl. Phys. Lett.90, 091109 (2007); Photon-Assisted Breakdown, Negative Resistance, AndSwitching In A Quantum-Well Transistor Laser, A. James, G. Walter, M.Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 90, 152109 (2007); FranzKeldysh Photon-Assisted Voltage-Operated Switching of a TransistorLaser, A. James, N. Holonyak, M. Feng, and G. Walter, PhotonicsTechnology Letters, IEEE Volume: 19 Issue: 9 (2007); ExperimentalDetermination Of The Effective Minority Carrier Lifetime In TheOperation Of A Quantum-Well n-p-n Heterojunction Bipolar Light-EmittingTransistor Of Varying Base Quantum-Well Design And Doping, H. W. Then,M. Feng, N. Holonyak, Jr., and C. H. Wu, Appl. Phys. Lett. 91, 033505(2007); Charge Control Analysis Of Transistor Laser Operation, M. Feng,N. Holonyak, Jr., H. W. Then, and G. Walter, Appl. Phys. Lett. 91,053501 (2007); Optical Bandwidth Enhancement By Operation And ModulationOf The First Excited State Of A Transistor Laser, H. W. Then, M. Feng,and N. Holonyak, Jr., Appl. Phys. Lett. 91, 183505 (2007); Modulation OfHigh Current Gain (13>49) Light-Emitting InGaN/GaN HeterojunctionBipolar Transistors, B. F. Chu-Kung, C. H. Wu, G. Walter, M. Feng, N.Holonyak, Jr., T. Chung, J.-H. Ryou, and R. D. Dupuis, Appl. Phys. Lett.91, 232114 (2007); Collector Characteristics And The DifferentialOptical Gain Of A Quantum-Well Transistor Laser, H. W. Then, G. Walter,M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 91, 243508 (2007);Transistor Laser With Emission Wavelength at 1544 nm, F. Dixon, M. Feng,N. Holonyak, Jr., Yong Huang, X. B. Zhang, J. H. Ryou, and R. D. Dupuis,Appl. Phys. Lett. 93, 021111 (2008); Optical Bandwidth Enhancement OfHeterojunction Bipolar Transistor Laser Operation With An Auxiliary BaseSignal, H. W. Then, G. Walter, M. Feng, and N. Holonyak, Jr. Appl. Phys.Lett. 93, 163504 (2008); Bandwidth Extension By Trade-Off Of ElectricalAnd Optical Gain In A Transistor Laser: Three-Terminal Control, H. W.Then, M. Feng, and N. Holonyak, Jr. Appl. Phys. Lett. 94, 013509 (2009);Tunnel Junction Transistor Laser, M. Feng, N. Holonyak, Jr., H. W. Then,C. H. Wu, and G. Walter Appl. Phys. Lett. 94, 041118 (2009);Electrical-Optical Signal Mixing And Multiplication (2→22 GHz) With ATunnel Junction Transistor Laser, H. W. Then, C. H. Wu, G. Walter, M.Feng, and N. Holonyak, Jr. Appl. Phys. Lett. 94, 101114 (2009); ScalingOf Light Emitting Transistor For Multigigahertz Optical Bandwidth, C. H.Wu, G. Walter, H. W. Then, M. Feng, and N. Holonyak, Jr. Appl. Phys.Lett. 94, 171101 (2009). Device Performance Of Light EmittingTransistors With C-Doped And Zn-Doped Base Layers, Huang, Y., Ryou,J.-H., Dupuis, R. D., Dixon, F., Holonyak, N., Feng, M., IndiumPhosphide & Related Materials, 2009; IPRM '09. IEEE InternationalConference, 10-14 May 2009, Pages 387-390; Tilted-Charge High Speed (7GHz) Light Emitting Diode, G. Walter, C. H. Wu, H. W. Then, M. Feng, andN. Holonyak, Jr. Appl. Phys. Lett. 94, 231125 (2009); 4.3 GHz OpticalBandwidth Light Emitting Transistor, G. Walter, C. H. Wu, H. W. Then, M.Feng, and N. Holonyak, Jr. Appl. Phys. Lett. 94, 241101 (2009); andResonance-Free Frequency Response Of A Semiconductor Laser, M. Feng, H.W. Then, N. Holonyak, Jr., G. Walter, and A. James Appl. Phys. Lett. 95,033509 (2009).

In tilted-charge light-emitting devices, including those described inthe above-referenced patents, patent application publications, andpublished papers, trade-offs in device design arise when striving tomaximize light output, response time (speed) of operation, and ease offabrication.

It is among the objects of the present invention to address thesetrade-offs and other limitations of existing tilted-charge lightemitting devices and methods.

SUMMARY OF THE INVENTION

The high speed optical capability of a tilted charge device depends onthe ability of the device to maintain charge tilt in the active region(typically, base region) of the device. (A charge tilt is characterizedby a ramp in the device's energy diagram which has a small initial valueat the base-collector or base-drain junction. If there is a chargebuild-up at this junction, the advantageous charge tilt characteristicwill not exist.) A charge tilt is enabled by ensuring that minoritycharges that do not recombine in the regions of desired opticalrecombination (e.g. quantum well(s), quantum dots, etc.) are collectedor drained by a faster secondary mechanism (such as the collector of atransistor or the drain of a tilted-charge light emitting diode). Theintrinsic high speed capability of the device is this limited by thetime required to access to thus secondary mechanism, also known as thetransit time, τ_(t).

The transistor, a structure comprising an emitter, base and collector,can be an excellent device to use for material study. Emitter gain β, ametric of the ratio of the collector current Ic, to the base current, Ib(β=Ic/Ib), can also be given as the ratio of the base recombination timeover the base transit time τ_(B)/τ_(t). Therefore, for transistors ofcomparable junction characteristics, similar physical dimensions andbase resistivity, a lower beta device usually indicates smaller (hence,faster) base recombination lifetime.

By using the transistor technique to study the recombination lifetime ofa base material, Applicant found that the defect levels in a highlydoped semiconductor material can be controlled through growth variationssuch as doping concentration, gas flow and growth temperature. Byincreasing the defect levels in a layer, the recombination speed of thatlayer increases, resulting in a device with much lower β. Applicant hasalso found that alloys such as AlGaAs are more prone to such decrease inrecombination lifetime when compared to binary systems such as GaAs. (Aswill be treated hereinbelow, diffusion length for minority carriers in asemiconductor material is inversely related to defect concentration inthe material.) Applicant also found that transistors with 13 as low as0.01 (that is, 99% of minority carrier recombined within a single passof the base region) could be achieved, indicating that a very fastnon-radiative recombination material is possible via defect engineering.Importantly, the engineering of defects can be done without degradingthe majority carrier electrical characteristics (resistivity) of thelayer. This fast recombination characteristic indicates that anengineered high defect layer (or engineered short diffusion lengthlayer) may be used as an effective secondary mechanism forcollection/draining of excess minority carriers. Also, combinations oneor more of engineered high defect concentration layers (short diffusionlength layers) and one or more engineered low defect concentrationlayers (long diffusion length layers) in design of a tilted-chargelight-emitting devices can provide substantial advantage.

As was noted above, diffusion length for minority carriers in asemiconductor material is inversely related to defect concentration inthe material. Although either defect concentration or its inverse,diffusion length, can be used in describing certain layers employed inthe invention, diffusion length will be the metric that is primarilyused in the subsequent description and claims hereof. It will beunderstood throughout, however, that defect concentration, employed inthe inverse sense, is an implied alternative.

A form of the invention is applicable for use in conjunction with alight-emitting semiconductor structure that includes a semiconductoractive region of a first conductivity type containing a quantum sizeregion and having a first surface adjacent a semiconductor input regionof a second conductivity type that is operative, upon application ofelectrical potentials with respect to said active and input regions, toproduce light emission from said active region. A method is provided forenhancing operation of said light-emitting semiconductor structure,comprising the following steps: providing a semiconductor output regionthat includes a semiconductor auxiliary layer of said first conductivitytype adjacent a second surface, which opposes said first surface of saidactive region, and providing said auxiliary layer as comprising asemiconductor material having a diffusion length for minority carriersof said first conductivity type material that is substantially shorterthan the diffusion length for minority carriers of the semiconductormaterial of said active region.

In an embodiment of this form of the invention, the step of providingsaid output region further includes providing a semiconductor drainregion of said second conductivity type adjacent said semiconductorauxiliary layer.

In another embodiment of this form of the invention, the step ofproviding said output region further includes providing a semiconductorcollector region of said second conductivity type adjacent saidsemiconductor auxiliary layer.

Also, in an embodiment of this form of the invention, the step ofproviding said output region that includes a semiconductor auxiliarylayer adjacent said second surface of said active region comprisesproviding said auxiliary layer of a semiconductor material havingsubstantially the same elemental constituents as the semiconductormaterial of said second surface of said active region. In one embodimenthereof, both of said semiconductor materials are substantially GaAs, butwith respective different concentrations of defects (and, accordingly,different diffusion lengths for minority carriers).

In another form of the invention, a method is set forth for producinglight emission from a semiconductor structure, including the followingsteps: providing a semiconductor structure that includes a semiconductorbase region of a first conductivity type and having a relatively longminority carrier diffusion length characteristic, between asemiconductor emitter region of a second conductivity type opposite tothat of said first conductivity type, and a semiconductor drain regionof said second conductivity type; providing, between said base regionand said drain region, a semiconductor auxiliary region of said firstconductivity type and having a relatively short minority carrierdiffusion length characteristic; providing, within said base region, aregion exhibiting quantum size effects; providing an emitter electrodecoupled with said emitter region; providing a base/drain electrodecoupled with said base region and said drain region; and applyingsignals with respect to said emitter and base/drain electrodes to obtainlight emission from said semiconductor structure.

In an embodiment of this form of the invention, the step of providing abase/drain electrode comprises providing said base/drain electrodecoupled with said base region, said auxiliary region, and said drainregion. In this embodiment, the first conductivity type is p-type andthe second conductivity type is n-type, said step of providing saidsemiconductor base region comprises providing a p-type base regionhaving an average doping concentration of at least about 10¹⁹/cm³ andsaid step of providing said auxiliary layer comprises providing p-typematerial having an average doping concentration of at least about10¹⁹/cm³.

In a further form of the invention, a method set forth for producinglight emission from a semiconductor structure, including the followingsteps: providing a transistor that includes a semiconductor base regionof a first conductivity type and having a relatively long minoritycarrier diffusion length characteristic, between a semiconductor emitterregion of a second conductivity type, opposite to that of said firstconductivity type, and a semiconductor collector region of said secondconductivity type; providing, between said base region and saidcollector region, a semiconductor auxiliary region of said firstconductivity type and having a relatively short minority carrierdiffusion length characteristic; providing, within said base region, aregion exhibiting quantum size effects; providing an emitter electrodecoupled with said emitter region, a base electrode coupled with saidbase region, and a collector electrode coupled with said collectorregion; and applying signals with respect to said emitter, base, andcollector electrodes to obtain light emission from said semiconductorstructure.

Also set forth is an embodiment of a semiconductor light-emittingdevice, comprising: a semiconductor active region of a firstconductivity type containing a quantum size region and having a firstsurface adjacent a semiconductor input region of a second conductivitytype; a semiconductor output region that includes a semiconductorauxiliary layer of said first conductivity type adjacent a secondsurface, which opposes said first surface, of said active region, saidauxiliary layer comprising a semiconductor material having a diffusionlength for minority carriers of said first conductivity type materialthat is substantially shorter than the diffusion length for minoritycarriers of the semiconductor material of said active region; whereby,application of electrical potentials with respect to said active andinput regions produces light emission from the active region of saidsemiconductor structure. In one embodiment, the output region furthercomprise a semiconductor drain region of said second conductivity typeadjacent said semiconductor auxiliary layer, and in another embodiment,the output region further comprise a semiconductor collector region ofsaid second conductivity type adjacent said semiconductor auxiliarylayer.

Further features and advantages of the invention will become morereadily apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional diagram, not to scale, of layersand regions of a tilted-charge light-emitting device that is useful indescribing operation of embodiments of the invention that employ layersof engineered long diffusion length (ELDL) and engineered shortdiffusion length (ESDL) layers to obtain improved operation.

FIG. 2 is an energy band diagram of the type of device in FIG. 1 thatuseful in understanding operation of embodiments of the invention.

FIG. 3 is a cross-sectional view showing the layer structure (not toscale) of an embodiment of a tilted-charge light-emitting device inaccordance with an embodiment of the invention.

FIG. 4 is a cross-sectional view showing the layer structure (not toscale) of another embodiment of a tilted-charge light-emitting device inaccordance with an embodiment of the invention.

FIG. 5 is a cross-sectional view showing the layer structure (not toscale) of a further embodiment of a tilted-charge light-emitting devicein accordance with an embodiment of the invention.

FIG. 6 is a cross-sectional view showing the layer structure (not toscale) of another embodiment of a tilted-charge light-emitting device inaccordance with an embodiment of the invention.

FIG. 7 is a cross-sectional view showing the layer structure (not toscale) of a further embodiment of a tilted-charge light-emitting devicein accordance with an embodiment of the invention.

FIG. 8 is a graph showing an example of a characteristic of diffusionlength of minority carriers, as a function of processing temperature.

FIG. 9 a graph of showing an example of emitter current gain as afunction of minority carrier diffusion length.

DETAILED DESCRIPTION

The diffusion length (L) of a minority carrier in the material of asemiconductor device is given as:

L=√{square root over (D×τ)}

where, D, is the diffusion coefficient, which depends on the carriermobility in the semiconductor, and μ is expressed in:

$D = {\mu \times \frac{kT}{q}}$

where q is the charge, T is temperature, and k is the Boltzmannconstant. The carrier mobility, μ, is proportional to the averagescattering time, which is dependent, among many factors, on dopingconcentrations, defect concentrations, and semiconductor materialcomposition (for example, GaAs versus AlGaAs (binary vs. alloy), or GaAsvs. InP (different material systems)). The minority carrier lifetime, τ,is also dependent, among other factors, on free carrier concentrations(related to doping concentrations), defect energy levels, and defectconcentration. In regions of short minority carrier diffusion lengths,minority carriers have higher probability of recombining with majoritycarriers per unit distance.

A tilted-charge device has an active region with built-in free majoritycarriers of one polarity, and on one input to this active region, onlyone species of minority carriers of another polarity are injected andallowed to diffuse across the active region. This active region hasfeatures that enhance the conduction of majority carriers and therecombination of minority carriers. On the output side of the region,minority carriers are then collected, drained, depleted or recombined bya separate and faster mechanism. Electrical contacts are coupled to thisfull-featured region.

An embodiment hereof employs a short minority diffusion length layer ina tilted charge device, for example, a light emitting transistor, atilted charge light emitting diode, or a transistor laser. In thisembodiment, the active region comprises doped layers engineered to haverelatively long minority carrier diffusion length (ELDL), and quantumsize region(s) for optical recombination. In the preferred embodiment,an engineered relatively short minority carrier diffusion length (ESDL)layer is provided after the active region, in the output region of thetilted charge device. The ESDL and ELDL layers are doped (directly orindirectly) to be of similar conductivity type (for example, a p-typematerial).

The technique of embodiments hereof allows the conductivity of majoritycarriers to be increased without increasing the active region thickness,in situations where a small active region is preferred for higher speedoperation. This also enables the use of very small active regions (forexample, less than about 25 nm) while still having the necessarythicknesses to reliably couple an electrical contact, and hencetransport majority carriers to the active region.

The simplified diagram of FIG. 1 illustrates the advantageous use ofengineered long diffusion length (ELDL) and engineered short diffusionlength (ESDL) layers in a tilted charge light-emitting semiconductordevice. In the FIG. 1 diagram, the input region of the device is emitter110. The active region of the device includes one or more quantum wells122 between undoped or low doped barriers within engineered longdiffusion length (ELDL) layers 121, 123. The output region of the deviceincludes engineering short diffusion length (ESDL) layer 131 and anelectrical collector 132, which may be, for example the collector of alight-emitting transistor or the drain of a tilted charge light-emittingdiode. The active region of this tilted charge device has abundantbuilt-in carriers (e.g. holes in a highly doped p-type region) and aninput region where minority carriers are injected into the activeregion. In the output region, minority carriers are depleted and/orrecombined and/or collected via a faster mechanism than the activeregion. As will be described hereinbelow, electrodes can be applied tothe input and output regions for two terminal operation, and also to theactive region for three terminal operation.

In order to improve the speed a tilted charge device, the transit time,τ_(T), has to be optimized. Since the transit time is proportional tothe square of width, W_(transit) ² (among other factors such asdiffusion constants) of the region it is transiting, the overall baseregion (W_(base)==W_(transit)) is generally made thin. Thus, forexample, in an optical tilted-charge device with an n-type emitter, thisleads to relatively large lateral resistances (high resistivity) for theconduction of holes in the p-type base region. Such large resistancestend to limit the operation of the device to small areas along the edgeof the emitter mesa. In an embodiment of the present invention, asrepresented in FIG. 1, the transit time can be maintained, whilelowering the resistivity of the overall p-type region (including thebase), by introducing the auxiliary layer (131) of engineered shortdiffusion length (ESDL) of low resistivity p-type material, which actsas a secondary mechanism that collects/drains and eliminates excessmajority carriers. This effectively increases the base width withoutincreasing the transit width (that is, W_(base)>W_(transit)). The p-typematerial preferably has a doping concentrations (e.g. carbon doping) ofat least 1E19 cm⁻³. Thereafter, a further collection or drain mechanismmay be included. It will be understood that the same principle can alsobe applied to optical tilted-charge devices with p-type emitters andn-type bases, where the doping concentration of the ESDL auxiliary layeris at least 1E18 cm⁻³.

FIG. 2 illustrates the type of energy band diagram that characterizesembodiments of the FIG. 1 structure. In the example of FIG. 2, region121, adjacent the emitter, can employ a relatively higher bandgapmaterial than region 123 to form an asymmetric base (see copending U.S.Patent Application Publication No. US2010/0202484). As previouslyindicated, the material of layer 121 is engineered long diffusion length(ELDL) for minority carriers. Region 122 contains one or more quantumwells, with undoped or low doped barriers, and the layer 123 portion ofthe active region is also ELDL. The auxiliary layer 131 is engineeredshort diffusion length (ESDL) material with similar or lower bandgapthan region 123. As previously described, since the relatively shortdiffusion length (higher defect concentration) material acts as asecondary mechanism that collects/drains majority carriers, the region131 does not substantially increase the transit width W_(transit), whileserving to provide additional low resistivity paths for majoritycarriers (i.e., increased W_(base)).

The growth of a semiconductor epilayer, for example by methods of metaloxide chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE),requires precise control of several variables including gas flow rates,growth rates, growth temperature and vacuum. Post-growth processes mayalso affect the resulting overall material characteristics of thesemiconductor. In one example hereof, the material quality may be tunedor optimized by varying the growth temperature of the semiconductorwhile keeping other variables as constant as possible. Such tuningresults in a typical epilayer with diffusion length characteristics asshown in FIG. 8. (There are several known methods to measure theminority carrier diffusion length. One method, which was notedhereinabove, is to embed the studied layer as the base region of aheterojunction bipolar transistor structure.) FIG. 8 is a graph ofdiffusion length of minority carriers (electrons) in p-type GaAsmaterial as a function of temperature variation. The resulting materialhas approximately the same sheet resistance. (In this example, there wasa dopant concentration of ˜4E19 cm⁻³, and sheet resistance of ˜165Ohm/sq.)

FIG. 8 shows that there exists an optimum growth temperature, for aparticular set of conditions, to maximize the material's diffusionlength of minority carriers. By detuning from this optimum temperature,the diffusion length can be shortened with only small changes in thematerial's sheet resistance. In this example, an MOCVD process is usedwith a selected gas flow rate for a particular growth rate and vacuumsetting, and the growth temperature is tuned to obtain an epilayerhaving the desired diffusion length characteristic. Thus, for example,to obtain a relatively long minority carrier diffusion length p-typeGaAs semiconductor (it being understood that post-growth doping andannealing processes can also be employed, as appropriate), a growthtemperature of about 580 degrees C. can be used to obtain substantiallythe maximum obtainable diffusion length, as seen in the diagram of FIG.8. This can be the manner of forming, for example, the ELDL layers 121,123 of the FIG. 1 example. When it is desired to grow an epilayer ofengineered short diffusion length (ESDL) material in this example, atemperature of, say, 555 degrees C. or 605 degrees C. could be used toobtain a diffusion length of minority carriers that is about half themaximum obtainable diffusion length; that is, ESDL material (e.g. forlayer 131 of FIG. 1) which has about half the diffusion length of theELDL material of layers 121, 123. Preferably, an ESDL material will havea diffusion length that is less than about 0.7 times the material'slayer thickness, whereas an ELDL material will have a diffusion lengththat is greater than the material's layer thickness.

FIG. 9 shows how the emitter current gain of a InGaP/GaAs HBT with abase thickness of 100 nm and sheet resistance of about 165 Ohm sq.,changes as a function of diffusion length. This demonstrates that longerdiffusion length material exhibits proportionally higher β and,accordingly, less recombination of minority carriers.

FIG. 3 illustrates an embodiment of a tilted charge light emitting diodethat employs a p-type engineered short diffusion length (ESDL) auxiliarylayer, for bottom light emission. In this example, a GaAs buffer layer315 is first grown on an undoped GaAs substrate 310. This buffer layercan be undoped or a p-type engineered long diffusion length layer. Next,a p-type GaAs engineered short diffusion length layer (ESDL) 320, whichwill serve as a drain, is grown on the buffer layer. Next, a p-type GaAsbase-2 region 330 is grown as an engineered long diffusion length layer330. A single or multiple quantum well region 340 is grown with undopedor low doped barrier layers. In this example, the quantum well(s) areInGaAs with GaAs barriers. The base region is then completed, as base-1region with GaAs p-type ELDL layer. The base-1 region may or may not beof larger bandgap than the base-2 region (symmetric vs. asymmetric basedesign; see, again, the copending U.S. Patent Application PublicationNo. US2010/0202484). A relatively large bandgap InGaP or InAlGaP n-typeemitter 370 is then grown, followed by an n-type emitter cladding layer380 which comprises a contact layer and an optional oxidizable AlGaAslayer to form an electrical confining aperture. Ti—Pt—Au or AuGe is thenmetalized on the exposed surface of the p-type base region to formcontact 352 to the p-type base material. This is followed by AuGecontact metallization 382 on the emitter mesa which also functions as amirror to reflect light downward. Finally, a collimator or focusing lens305 is then molded or affixed to the thinned down GaAs substrate. Thedevice is operated under forward bias conditions, where the base biasvoltage, VB and emitter bias voltage, VE, is biased so that VBE>EQW,where EQW is the energy gap of the quantum well. A partial DBR or fullDBR cavity may also be incorporated into this structure. Thisembodiment, and others hereof, can also be operated as lasers byproviding suitable resonant optical cavities.

Referring to FIG. 4, there is shown an embodiment of a tilted chargelight-emitting diode that employs a p-type engineered short diffusionlength (ESDL) for top light emission. In this embodiment, the layers310, 315, 320, 340, 315, 320, 340, 350, and 370, and the contact 352 canbe similar to their counterparts of like reference numerals in FIG. 3.For top light emission, the present embodiment employs an oxide aperturedefined by annular oxidized region 483 in n-type emitter cladding region480. A bottom distributed Bragg reflector (DBR) 412 is used to reflectlight upward. An optional upper DBR (low reflectivity for spontaneousoperation and high reflectivity for laser operation) can be embedded inthe emitter cladding layer 480 for resonant cavity design. The emittercontact metallization 482 is in the form of an annular ring. Acollimating or focusing lens is then molded or affixed onto the topsurface. Operation can again be in forward biased mode.

FIG. 5 shows an embodiment of a tilted-charge device in the form of athree terminal light-emitting transistor incorporating both anengineered short diffusion length (ESDL) auxiliary/drain region and ahigh impedance collector. In this embodiment, the device comprisesundoped GaAs substrate 510, n-type subcollector 520 with contact 522,high impedance (undoped) collector 525, and p-type ESDL auxiliary/drainlayer 540. The layers 330, 340, 350, 370, and 380, and the contacts 352and 382 can be similar to their counterparts of like reference numeralsin FIG. 3. The device can be operated in common base, common collector,or common emitter mode.

FIG. 6 shows an embodiment of a tilted-charge light-emitting diode withan engineered short diffusion length (ESDL) layer. In this embodiment,the base-2 layer 330, quantum wells, with buffer region 340, base-1layer 350, emitter layer 370, emitter lading 382 and emitter contact, aswell as the undoped GaAs substrate 310 and the bottom collimating orfocusing lens 305, can be similar to their counterparts of likereference numerals in the FIG. 3 embodiment. In the present embodiment,an n-type subdrain layer 620 is grown on the substrate, and then,undoped drain layer 622. Deposited on the drain layer 622 is auxiliarydrain layer 625, which is grown as an engineered short diffusion length(ESDL) layer. The deposition of the further layers, as was previouslydescribed, is implemented, with the mesas being formed as shown. Then,the metalizations are formed for emitter contact 382 and base/draincontact 392, which has annular upper and lower portions that contact therespective shelves of base layer 350 and subdrain layer 620, and sidesthat contact the peripheral edges of the intervening layers.

FIG. 7 shows an embodiment of another tilted-charge light-emitting diodewith an embedded engineered short diffusion length (ESDL) layer. Thegeneral configuration and layer structure is similar to that of the FIG.6 embodiment (as indicated by like reference numerals indicatingcorresponding elements), but the FIG. 7 embodiment has tunnel Junction722 in place of the FIG. 6 drain layer 622. The tunnel junction 722comprises heavily doped (p⁺⁺) region 723 adjacent heavily doped (n⁺⁺)region 724. (Reference can be made to U.S. Patent ApplicationPublication No. US2010/0202483, for description of a tilted-chargelight-emitting device employing a tunnel junction.) In operation, theESDL auxiliary drain layer 625 serves to reduce the avalanche currentacross the tunnel junction by reducing the base-drain electron currentflow (as represented in the diagram by the narrowing arrow width).

1. For use in conjunction with a light-emitting semiconductor structurethat includes a semiconductor active region of a first conductivity typecontaining a quantum size region and having a first surface adjacent asemiconductor input region of a second conductivity type that isoperative, upon application of electrical potentials with respect tosaid active and input regions, to produce light emission from saidactive region, a method for enhancing operation of said light-emittingsemiconductor structure, comprising: providing a semiconductor outputregion that includes a semiconductor auxiliary layer of said firstconductivity type adjacent a second surface, which opposes said firstsurface of said active region, and providing said auxiliary layer ascomprising a semiconductor material having a diffusion length forminority carriers of said first conductivity type material that issubstantially shorter than the diffusion length for minority carriers ofthe semiconductor material of said active region.
 2. The method asdefined by claim 1, wherein said step of providing said output regionfurther includes providing a semiconductor drain region of said secondconductivity type adjacent said semiconductor auxiliary layer.
 3. Themethod as defined by claim 1, wherein said step of providing said outputregion further includes providing a semiconductor collector region ofsaid second conductivity type adjacent said semiconductor auxiliarylayer.
 4. The method as defined by claim 3, further comprising applyingelectrical potential to said collector region with respect to the otherregions.
 5. The method as defined by claim 1, wherein said step ofproviding said output region that includes a semiconductor auxiliarylayer adjacent said second surface of said active region comprisesproviding said auxiliary layer of a semiconductor material havingsubstantially the same elemental constituents as the semiconductormaterial of said second surface of said active region.
 6. The method asdefined by claim 5, wherein said step of providing said auxiliary layerof a semiconductor material having substantially the same elementalconstituents as said second surface of said active region comprisesproviding both of said semiconductor materials as substantially GaAs. 7.The method as defined by claim 1, further comprising disposing saidactive region in an optical resonant cavity, and wherein said lightemission is laser emission.
 8. A method for producing light emissionfrom a semiconductor structure, comprising the steps of: providing asemiconductor structure that includes a semiconductor base region of afirst conductivity type and having a relatively long minority carrierdiffusion length characteristic, between a semiconductor emitter regionof a second conductivity type opposite to that of said firstconductivity type, and a semiconductor drain region of said secondconductivity type; providing, between said base region and said drainregion, a semiconductor auxiliary region of said first conductivity typeand having a relatively short minority carrier diffusion lengthcharacteristic; providing, within said base region, a region exhibitingquantum size effects; providing an emitter electrode coupled with saidemitter region; providing a base/drain electrode coupled with said baseregion and said drain region; and applying signals with respect to saidemitter and base/drain electrodes to obtain light emission from saidsemiconductor structure.
 9. The method as defined by claim 8, whereinsaid step of providing a base/drain electrode comprises providing saidbase/drain electrode coupled with said base region, said auxiliaryregion, and said drain region.
 10. The method as defined by claim 9,further comprising providing an undoped semiconductor region betweensaid auxiliary region and said drain region.
 11. The method as definedby claim 9, wherein said first conductivity type is p-type and saidsecond conductivity type is n-type.
 12. The method as defined by claim11, wherein said step of providing said semiconductor base regioncomprises providing a p-type base region having an average dopingconcentration of at least about 10¹⁹/cm³ and said step of providing saidauxiliary layer comprises providing p-type material having an averagedoping concentration of at least about 10¹⁹/cm³.
 13. The method asdefined by claim 9, wherein said first conductivity type is n-type andsaid second conductivity type is p-type.
 14. The method as defined byclaim 13, wherein said step of providing said semiconductor base regioncomprises providing an n-type base region having an average dopingconcentration of at least about 10¹⁸/cm³, and said step of providingsaid auxiliary layer comprises providing n-type material having anaverage doping concentration of at least about 10¹⁸/cm³.
 15. The methodas defined by claim 9, where in said step of providing saidsemiconductor base region comprises providing a first base regionportion on a side of said quantum size region that is adjacent saidemitter region and providing a second base region portion on a side ofsaid quantum size region that is adjacent said auxiliary region, andwherein said first base region portion is provided as a semiconductormaterial having a higher bandgap than said second base region portion.16. The method as defined by claim 9, wherein said step of providingsaid region exhibiting quantum size effects comprises providing at leastone quantum well.
 17. The method as defined by claim 8, furthercomprising providing a tunnel junction in conjunction with said drainregion.
 18. A method for producing light emission from a semiconductorstructure, comprising the steps of: providing a transistor that includesa semiconductor base region of a first conductivity type and having arelatively long minority carrier diffusion length characteristic,between a semiconductor emitter region of a second conductivity type,opposite to that of said first conductivity type, and a semiconductorcollector region of said second conductivity type; providing, betweensaid base region and said collector region, a semiconductor auxiliaryregion of said first conductivity type and having a relatively shortminority carrier diffusion length characteristic; providing, within saidbase region, a region exhibiting quantum size effects; providing anemitter electrode coupled with said emitter region, a base electrodecoupled with said base region, and a collector electrode coupled withsaid collector region; and applying signals with respect to saidemitter, base, and collector electrodes to obtain light emission fromsaid semiconductor structure.
 19. The method as defined by claim 18,wherein said first conductivity type is p-type and said secondconductivity type is n-type.
 20. The method as defined by claim 19,wherein said step of providing said semiconductor base region comprisesproviding a p-type base region having an average doping concentration ofat least about 10¹⁹/cm³ and said step of providing said auxiliary layercomprises providing p-type material having an average dopingconcentration of at least about 10¹⁹/cm³.
 21. The method as defined byclaim 19, wherein said first conductivity type is n-type and said secondconductivity type is p-type.
 22. The method as defined by claim 21,wherein said step of providing said semiconductor base region comprisesproviding an n-type base region having an average doping concentrationof at least about 10¹⁸/cm³, and said step of providing said auxiliarylayer comprises providing n-type material having an average dopingconcentration of at least about 10¹⁸/cm³.
 23. A semiconductorlight-emitting device, comprising: a semiconductor active region of afirst conductivity type containing a quantum size region and having afirst surface adjacent a semiconductor input region of a secondconductivity type; a semiconductor output region that includes asemiconductor auxiliary layer of said first conductivity type adjacent asecond surface, which opposes said first surface, of said active region,said auxiliary layer comprising a semiconductor material having adiffusion length for minority carriers of said first conductivity typematerial that is substantially shorter than the diffusion length forminority carriers of the semiconductor material of said active region;whereby, application of electrical potentials with respect to saidactive and input regions produces light emission from the active regionof said semiconductor structure.
 24. The device as defined by claim 23,wherein said output region further comprise a semiconductor drain regionof said second conductivity type adjacent said semiconductor auxiliarylayer.
 25. The device as defined by claim 23, wherein said output regionfurther comprise a semiconductor collector region of said secondconductivity type adjacent said semiconductor auxiliary layer.